1. Field of the Invention
The present invention relates to an interface circuit which can be used in a RS232C or RS232D serial interface circuit adapted to transmit and receive serial data, and more specifically to such an interface circuit comprising as an electric voltage source a DC/DC converter configured to generate positive and negative elevated voltages from a single electric power source.
2. Description of Related Art
Conventionally, a single power source type interface circuit includes, as an electric voltage supplying source, a DC/DC converter, which is now mainly of a so called charge pump type.
This charge pump type DC/DC converter is configured to be driven with a clock signal. When this DC/DC converter is incorporated in a data terminal instrument, the interface is constructed such that in case of no data transmission, an oscillator supplying the clock to the DC/DC converter is stopped so as to cause the charge pump type DC/DC converter to stop its function, for the purpose of saving the power consumption.
On the other hand, in portable data terminal instruments such as a handy terminal driven with a battery cell, a consumed electric power is an important problem in a real time communication. Because of this reason, it is ideally desired to stop the DC/DC converter in case of no communication and to immediately start the DC/DC converter at the time of transmitting data so that a data communication is rapidly performed. However, in the DC/DC converter incorporated in the conventional serial interface circuit for RS232C and others, since the oscillation frequency of the oscillator is fixed to one kind, both of a delay of a supply starting time of the elevated output voltage and an increase of the consumed electric power could not overcome, since they are demands conflicting with each other.
Namely, if the oscillation frequency is high, a through current increases in the DC/DC converter which is generally constituted of CMOS circuits, and therefore, the consumed electric power increases. This is a fatal problem in the battery cell driven device such as the handy terminal.
On the other hand, if the oscillation frequency is low, an electric charge accumulation is delayed in a charge pump circuit of the DC/DC converter, and therefore, the supply start of the elevated output voltage is delayed. Namely, a real time communication cannot be executed until the elevated output voltage becomes stable. In this case, it was a conventional practice to cause the DC/DC converter to ceaselessly operate in the case of the real time communication.